New chamber walls conditioning and cleaning strategies to improve the stability of plasma processes

Abstract

One major challenge in plasma etching processes for integrated circuit fabrication is to achieve a good wafer-to-wafer repeatability. This requires a perfect control of the plasma chamber wall conditions. For silicon etching processes, which deposit SiO y Cl z layers on the chamber walls, this is achieved by cleaning the interior surfaces of the plasma chamber with an SF 6 -based plasma after each wafer is etched. However, x-ray photoelectron spectroscopy analysis of the reactor wall surfaces shows that the inner parts of the Al 2 O 3 chamber are strongly fluorinated (formation of Al-F bonds) during the SF 6 plasma. At the same time the AlF x layer is sputtered from some parts of the chamber (mostly from the roof, which is bombarded by high energy ions), and AlF redeposition is observed on other parts of the reactor body. Hence, the cleaning process of the reactor leaves AlF residues on the chamber wall on its own. This leads to several issues including flake off of Al x F y particles on the wafer and process drifts (due both to the progressive growth of AlF material on the SiO 2 windows and to the release of F atoms from the chamber walls during the etching process). This indicates that a strategy other than dry-cleaning the Al 2 O 3 chamber walls in fluorine-based plasmas should be found. In this paper we have investigated two different strategies. The first one consists of replacing Al 2 O 3 covering the chamber walls by another material for the chamber walls inner coating. In particular, we have investigated the surface modification of several types of organic polymers (Teflon, Parylene and carbon-rich polymers), when exposed to SF 6 -based plasmas. We show that these materials can be reset to their original condition after exposure to a dry-cleaning process because carbon containing polymers are slowly etched away by the SF 6 /O 2 plasma. This suggests that the replacement of the conventional Al 2 O 3 chamber wall material by a carbon-coated liner should be possible. Alternatively, we also propose a powerful strategy for conditioning and cleaning an Al 2 O 3 reactor, in which a thin carbon-rich layer is deposited on the reactor walls by a short plasma step prior to any etching process. After etching, the SiO y Cl z layer deposited on the carbon layer during a silicon gate etch step can be cleared with an appropriate plasma, and the carbon layer removed by an O 2 plasma, thus resetting the reactor walls to their initial state. Using this strategy the etching process always starts under the same chamber walls conditions (a carbon-rich wall) and is thus reproducible. At the same time, the issues associated with AlF deposits are prevented because the carbon-coated layer protects the Al 2 O 3 chamber walls, and there is no fluorine released into the plasma. Finally, we will show that the etching profiles of the silicon gates and the selectivity towards the thin gate oxides are excellent in the carbon-coated chamber. This strategy is thus promising for future metal gate etching applications.

Overview of atomic layer etching in the semiconductor industry

“…23 In the early 2000s, conditioning chamber walls with in situ wall deposition significantly improved wafer-to-wafer repeatability. 24 However, a significant portion of process control still relies on compensation to correct for systematic nonuniformities that are amplified by the coupled transport of ions and neutrals to the wafer surface during continuous processing. Despite widespread usage of balancing competing behaviors, compensation adds cost and complexity to the system, thereby providing a smaller process window.…”

Chemical analysis of deposits formed on the reactor walls during silicon and metal gate etching processes

“…In recent papers 5,8 we have presented a technique, which allows us to measure by XPS the chemical nature and thickness of the layers deposited on the reactor walls after an etching process. As shown in Fig.…”

“…The empirical approach used in the industry has shown that the most effective way to maintain stable plasma processing conditions together with a high mean time between chamber ͑MTBC͒ wet cleans is to clean up the reactor walls after each wafer is etched. 7,8 However, it remains difficult to perfectly remove the coatings from all the parts of the chamber 9 because their chemical nature is unknown ab initio and may greatly vary depending on the processing conditions, on the photoresist coverage of the wafer, and on the material being etched. 5 It is thus important to analyze the chemical nature of the layers coated on the reactor walls after an etching process in order to develop efficient cleaning processes of the plasma chamber.…”

“…These studies have converged to show that silicon etching in Cl 2 /O 2 and HBr/ Cl 2 /O 2 plasmas result in the formation of silicon oxichloride layers on the chamber walls and that these layers can be cleared from the Al 2 O 3 walls by SF 6 -based plasmas. 8 However, these studies have been performed on blanket silicon wafers after a single step etching process. These conditions remain far from true industrial silicon gate etching processes, which include a sequence of many etching steps to pattern complex gate stacks.…”

Impact of metal etch residues on etch species density and uniformity

“…During the SF 6 / O 2 plasma treatment in our experiment, SiO 2 and AlF are deposited from the chamber walls on the wafer piece. Cunge et al 9 showed that even in a SF 6 / O 2 plasma AlF can be released from the walls into the plasma and eventually deposit on the floating piece. The AlF 3 on the piece is thus not only coming from fluorination of the Al 2 O 3 piece and the piece really represents the actual chamber wall condition during the plasma treatment.…”